In general, IFF equipment does not keep track of which Identification Friend or Foe (IFF) targets are given a chance to respond to cryptographic (e.g., Mode IV) IFF interrogations, nor how many chances each unknown target is given to reply correctly.
Existing IFF equipment generally provides an output signal FA if a target responds properly, and a different output signal 421 if it does not--but the operator must remember how many times he (or his equipment) has attempted to identify each target. The absence of any precise record of how many opportunities each target has been given to respond can easily result in giving a "spoofing" enemy far too many chances to guess acceptable replies. The direct result is that the "enemy acceptance criterion" generally specified for the equipment may not be satisfied.
Further, it is desirable to automatically limit the number of cryptographic interrogations that unknown target is given in accordance with some predetermined limit. There is, in general, no IFF equipment today that retains information concerning targets obtained on one scan for use on another scan. Generally, information obtained on a scan is used to make a decision, and the information is then erased.